URANUS (MYTHOLOGY)
In Greek mythology, Uranus represented the sky or heaven. He
was both the son and husband of GAEA, the earth, by whom he
fathered first the hundred-handed giants and the CYCLOPES, whom
he banished to Tartarus, and later the TITANS. Gaea, angry
because her children were imprisoned, set the Titans against
Uranus. CRONUS, their leader, castrated Uranus and succeeded
him as ruler of the universe. According to Hesiod's Theogony
APHRODITE was born of the foam of Uranus's discarded genitals
as they fell in the sea.
Uranus
Green with jealousy
because the plankton sea
of Earth
endlessly
paints miracles on canvasses Uranus cannot see
for the sun hides
microscopically
the portraits of teeming life
in cosmic mystery.
carlyle miller

Uranus

Uranus

Uranus: Oberon

Uranus: Umbriel

URANUS (PLANET)
Uranus is the seventh PLANET from the Sun and was the first to
be discovered since ancient times. Sir William HERSCHEL first
observed the planet on Mar. 13, 1781, seeing a featureless
bluish green disk that he nevertheless recognized as a highly
unusual object. Although Herschel wished to call the newly
discovered planet Georgium Sidus (Georgian Star) for King
George III of England, and although many French astronomers
referred to it as Herschel, Johann Bode's proposal of the name
Uranus--the mythological father of Saturn--was over the years
accepted more and more widely and finally became universal in
the mid-19th century.
The only spacecraft to encounter Uranus thus far, VOYAGER 2,
collected data on the planet and its rings and satellites over
a four-month period, between Nov. 4, 1985, and Feb. 25, 1986.
Voyager 2 passed within 107,000 km (66,500 mi) of the center of
Uranus--about 81,450 km (50,625 mi) above its cloud tops--on
Jan. 14, 1986.
Appearance
Although it was discovered with a telescope, Uranus reaches a
maximum brightness of magnitude 5.5 and can be seen by the
naked eye as a faint point of light in a clear, moonless sky.
In Voyager 2 photos, Uranus appears as a generally featureless
disk, except for considerable darkening toward the edges.
Faint banding is seen in contrast-enhanced images, as are
transitory cloud plumes.
Orbit and Rotation
Uranus's average distance from the Sun is 2.875 billion km
(1.786 billion mi), and the planet takes 84.013 Earth-years to
make one complete revolution about the Sun. The orbit has an
eccentricity of 0.0472 and an inclination of 0 deg 46 min. Its
period of rotation was determined by Voyager 2 to be 17.24
hours. One startling aspect of Uranus is that its rotation
axis is inclined 97 deg. 54 min. from its orbital plane.
This means that the poles of Uranus lie nearly in the plane of
its orbit around the Sun.
Physical Characteristics
Uranus has an equatorial diameter of 51,100 km (31,750 mi),
almost precisely four times that of the Earth. The dark
absorption bands discovered in 1869 in the red part of the
spectrum of Uranus were identified in 1932 as being caused by
methane gas in Uranus's atmosphere. This red absorption is the
major cause of the blue green color of the planet; the methane
absorbs much of the red light from the impinging white
sunlight, leaving a bluish green color in the reflected
sunlight. Measurements by Voyager 2 verified that hydrogen is
the main constituent of the visible atmosphere. Helium is the
other major constituent, about 15% in terms of molecular
weight, which is a larger percentage than in the atmospheres of
either Jupiter or Saturn. Together, hydrogen and helium make
up more than 99% of Uranus's atmosphere, which extends to a
depth of about 8,000 km (5,000 mi). A layer of methane ice
clouds was detected by Voyager 2 near a pressure level of about
one atmosphere. Deep within Uranus exists a superheated water
ocean, perhaps 10,000 km (6,000 mi) deep, which contains large
concentrations of ionized chemicals. It is in this ocean that
Uranus's magnetic field may originate. Beneath this ionic
ocean, an Earth-sized core of molten rocky materials is
believed to exist.
Infrared measurements from Voyager 2 indicate that Uranus has
an effective temperature of -214 deg. C (-353 deg. F).
Unlike the other giant planets, there is no evidence of any
significant internal heat source. A very extended corona of
atomic and molecular hydrogen encompasses all the known rings
of the planet, perhaps heated by low-energy electrons.
Sunlight falling on this corona causes it to radiate
ultraviolet light, a phenomenon that scientists have termed
ELECTROGLOW.
Rings
The discovery on Mar. 10, 1977, that Uranus has rings was one
of the most unexpected and exciting events of modern planetary
astronomy. Subsequent observations have indicated the presence
of 10 narrow rings of dark particles and one broad, diffuse
ring, in addition to 100 or more possibly transient ringlets of
dust-sized particles seen only in Voyager 2 images of the
backlighted rings.
The outermost ring (called the epsilon ring), in marked
contrast to the rings of Saturn, has almost no particles
smaller than about 20 cm (8 in). The other rings also seem to
be deficient in small particles. Possibly the extended
hydrogen corona exerts a drag on the orbiting ring particles
that preferentially removes the smaller particles, causing them
to fall into the planet. Because collisions between larger
particles create smaller particles, atmospheric-drag forces
could virtually destroy the rings in geologically short time
periods. The rings are therefore relatively young or else are
replenished by the breakup of small satellites.
Satellites
Uranus has five major satellites: MIRANDA, discovered by
Gerard Kuiper in 1948; Ariel and Umbriel, discovered by
William Lassell in 1851; and Titania and Oberon, discovered by
William Herschel in 1787. Ten small satellites were also
discovered in Voyager 2 photographs, all of them orbiting
Uranus well inside the orbit of Miranda. The five major
satellites--and probably the ten smaller ones as well--appear
to be in synchronous rotation; that is, they keep their same
faces toward Uranus as they orbit the planet. Much like the
ring particles, the smaller satellites appear to have surfaces
as dark as coal. The major satellites have somewhat brighter
surfaces, reflecting from 19% (Umbriel) to 40% (Ariel) of the
sunlight that falls on them. In addition to being the darkest
of the major satellites, Umbriel also has the fewest geological
features. The other four satellites display increasingly
complex geologies with decreasing distance from Uranus.
Oberon and Titania are remarkably similar in size, density
color, and reflectivity. Titania, however, has a much larger
fraction of small craters and more fractures across its
surface, implying that Titania has a geologically younger
surface. Oberon has one mountain that rises to an altitude of
at least 20 km (12 mi) above the surrounding surface.
Although Umbriel and Ariel have similar diameters and
densities, they differ dramatically in the appearance of their
surfaces. Ariel's surface is literally covered with fractures
and fault systems. Parallel fractures in several areas bound
valleys that appear to have glacierlike flows along their
floors. Water-ammonia ices become fluid at much lower
temperatures than pure water ice and might form the observed
glaciers.
Miranda is the innermost and smallest of the major moons. its
surface consists of an old, heavily cratered, rolling terrain
with relatively uniform reflectivity, and a younger, complex
terrain in three nearly rectangular regions characterized by
parallel grooves, an abundance of vertical structures, and
large brightness differences. The materials out of which
Miranda was formed may not have melded together to form a
uniform surface; its present bizarre appearance may be a
frozen record of the late stages of the development of this
satellite.
Bibliography: Burgess, Eric, Uranus and Neptune (1988);
Ingersoll, Andrew P., "Uranus," Scientific American, January
1987; Hunt, Garry, and Moore, Patrick, Atlas of Uranus (1989);
Johnson, Torrence V., et al., "The Moons of Uranus," Scientific
American, April 1987; Laeser, Richard P., et al., "Engineering
Voyager 2's Encounter with Uranus," Scientific American,
November 1986; O'Meary, S. J., "A Visual History of Uranus,"
Sky & Telescope, November 1985; Stone, E. C., and Miner, E.
D., "The Voyager 2 Encounter with the Uranian System," Science,
July 4, 1986.
SATELLITES AND RINGS OF URANUS
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Average Distance from Period of
Center of Uranus Revolution
Name km mi (days)
---------------------------------------------------------------
1986U2R ring 38,000 24,000
6 ring 41,850 26,000
5 ring 42,240 26,250
4 ring 42,580 26,460
Alpha ring 44,730 27,790
Beta ring 45,670 28,380
Eta ring 47,180 29,320
Gamma ring 47,630 29,600
Delta ring 48,310 30,020
Cordelia 49,700 30,900 0.33
1986U1R ring 50,040 31,090
Epsilon ring 51,160 31,790
Ophelia 53,800 33,400 0.38
Bianca 59,200 36,800 0.43
Cressida 61,800 38,400 0.46
Desdemona 62,700 39,000 0.48
Juliet 64,600 40,100 0.49
Portia 66,100 41,100 0.51
Rosalind 69,900 43,400 0.56
Belinda 75,300 46,800 0.62
Puck 86,000 53,400 0.76
Miranda 129,900 80,700 1.41
Ariel 190,900 118,600 2.52
Umbriel 266,000 165,300 4.15
Titania 436,300 271,100 8.70
Oberon 583,400 362,500 13.46
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Diameter Orbital Orbital
or Ring Width Inclination Eccentricity
km mi (degrees)
---------------------------------------------------------------
2,500 1,500 0? 0?
2 1 0.063 0.0010
3 2 0.052 0.0019
2 1 0.032 0.0011
10 6 0.014 0.0008
9 6 0.005 0.0004
2 1 0.002 0.000
3 2 0.011 0.000
6 4 0.004 0.000
40 25 0? 0?
2 1 0? 0?
58 36 0.001 0.0079
50 30 0? 0?
50 30 0? 0?
60 35 0? 0?
60 35 0? 0?
80 50 0? 0?
80 50 0? 0?
60 35 0? 0?
60 35 0? 0?
170 105 0? 0?
484 301 4.2 0.0027
1,160 720 0.3 0.0034
1,190 740 0.36 0.0050
1,610 1,000 0.14 0.0022
1,550 960 0.10 0.0008
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